Briefly, in the semiconductor industry, microlithography (hereinafter “lithography”) is the process of printing the integrated circuit (“IC”) patterns on a wafer (for example, a semiconductor wafer such as silicon or GaAs). Currently, optically based lithography is the predominant technology used in volume IC manufacturing. Such lithography employs light in the visible to deep ultraviolet spectrum range to expose the resist on wafer. Following exposure the resist is developed to yield a relief image.
In optical lithography, a photomask (often called mask or reticle) is first written using electron-beam or laser-beam direct-write tools. The mask contains certain patterns and features that are used to create desired circuit patterns on a wafer. The process of fabricating a complete integrated circuit typically requires the use of many masks.
A typical photomask for optical lithography consists of a glass (or quartz) plate of 6-8 inches on a side, with one surface coated with a thin metal layer (for example, chrome) of a thickness of about 100 nm. The pattern, which is representative of a portion of the integrate circuit, is etched into the metal layer, thereby allowing light to transmit or pass through. The area where the metal is not etched or removed blocks or inhibits light transmission. In this way, a pattern may be projected onto the wafer.
The tool currently used in projecting the mask image onto wafer is called a stepper or scanner (hereinafter collectively called “photolithographic equipment” or “stepper”). (See, FIG. 1). The photomask contains the circuit pattern to be imaged on the wafer by the reduction lens system.
At various stages or steps of IC manufacturing, the masks and wafers must be inspected in order to identify or detect defects that impact the integrity of the fabrication process. For example, the photomask defect inspection is used to ensure that masks are defect free. This is a critical inspection since any undetected mistake or defect has the potential of creating many worthless IC wafers. As such, mask inspection typically requires 100% defect capture or detection rate.
In contrast, wafer inspection does not require 100% defect capture or detection rate, since wafer inspection is used for process control such as process debugging, tuning, failure analysis, and fabrication statistical process control.
Conventional techniques for mask defect inspection include:                Die-to-database (“D:DB”), which compares a captured optical image (or images captured using other means (for example, an electron beam system). From this point on, the “optical image” may be used to generally refer to the image captured by the inspection system, which may use optical imaging method, or other method like electron beam) of the photomask with a rendered image from the intended design pattern database.        Die-to-die (“D:D”), which compares two optical images from different dice on the same mask, where the two dice are designed to have the same pattern. Since the probability that the same defect happens at the same spot of two dice is almost zero, an “agreement” between the two images is considered to indicate good pattern, and a difference is considered to reflect a defect.        
Conventional wafer inspection, however, nearly exclusively employs a D:D approach, because:                There is typically multiple dice on a wafer, so D:D is generally easy to implement.        Rendering multiple layers of database is extremely complex—generally, integrated circuits are comprised of multiple layers.        Wafer inspection is not intended to capture repeating defects (caused by defects on mask) between dice. Instead, wafer inspection is intended to identify random, generally non-repeating defects that occur during manufacturing.        
There are several challenges of employing D:DB inspection of photomasks. First, a D:DB inspection system and technique tends to require extremely large computation capabilities in preparation, rendering and processing the database images. Indeed, as integrated circuit dimensions shrink, and optical proximity correction (“OPC”) photomasks and phase shift photomasks (“PSM”) are employed, the number of patterns in a unit area increases tremendously. Currently, for example, the pattern density is in the millions of patterns per mm2. As such, the computational requirements to render the database image tend to explode as integrated circuit dimensions shrink and new technologies (for example, OPC photomasks and PSM) are implemented.
Moreover, the reduced pattern dimension introduces non-linearity effects in mask pattern generation and in image capturing. This requires even more computation overhead to process the database image before it can be compared with the captured optical image.
Second, to match the captured optical image with the database image tend to require the inspection system to have very high-precision alignment between the mask position and the database. This stringent precision may be compounded because of the desire for high throughput. In addition, there are also stringent requirements on the image acquisition system (for example, focus, vibration and/or distortion). These requirements, in the aggregate, cause the system hardware to be complex and expensive, and the setup and management thereof quite difficult.
For D:D inspection, some of the difficulties encountered in D:DB inspection are partially alleviated since there is no database image to be prepared and processed. However, D:D mask inspection techniques must still be concerned with matching the two captured images. This, as mentioned above, requires the inspection system to have very high-precision alignment between the two positions. In addition, the image acquisition system must be tightly controlled with respect to, for example, stage control, focus, stability and vibration.
Notably, one distinct disadvantage of D:D inspection is that it requires two or more dice to be present on the same mask. Large-die IC's tend to be manufactured with photomasks having only one die. Thus, D:D inspection may not be available for such large-die IC's (for example, advanced microprocessors and field programmable gate arrays (“FPGA”)).
Thus, there is a need for a new system and technique for defect inspection of, for example, photomasks and the lithographic processes. There is a need for a defect self-inspection system and technique (i.e., defect detection based on a captured optical image itself without comparing to a reference image, where the reference image is the database image or an optical image from another die).